Maintaining Consistent Photodetector Sensitivity in an Optical Measurement System

ABSTRACT

An illustrative optical measurement system includes a light source configured to emit light directed at a target. The optical measurement system further includes a photodetector configured to detect a photon of the light after the light is scattered by the target. The optical measurement system further includes a control circuit configured to arm the photodetector by applying a bias voltage to a first terminal of the photodetector and applying, for a predetermined amount of time using a current source, a current to a second terminal of the photodetector to produce a predetermined voltage difference across the photodetector.

RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. § 119(e) to U.S.Provisional Patent Application No. 62/992,529, filed on Mar. 20, 2020,and to U.S. Provisional Patent Application No. 63/074,721, filed on Sep.4, 2020. These applications are incorporated herein by reference intheir respective entireties.

BACKGROUND INFORMATION

Detecting neural activity in the brain (or any other turbid medium) isuseful for medical diagnostics, imaging, neuroengineering,brain-computer interfacing, and a variety of other diagnostic andconsumer-related applications. For example, it may be desirable todetect neural activity in the brain of a user to determine if aparticular region of the brain has been impacted by reduced bloodirrigation, a hemorrhage, or any other type of damage. As anotherexample, it may be desirable to detect neural activity in the brain of auser and computationally decode the detected neural activity intocommands that can be used to control various types of consumerelectronics (e.g., by controlling a cursor on a computer screen,changing channels on a television, turning lights on, etc.).

Neural activity and other attributes of the brain may be determined orinferred by measuring responses of tissue within the brain to lightpulses. One technique to measure such responses is time-correlatedsingle-photon counting (TCSPC). Time-correlated single-photon countingdetects single photons and measures a time of arrival of the photonswith respect to a reference signal (e.g., a light source). By repeatingthe light pulses, TCSPC may accumulate a sufficient number of photonevents to statistically determine a histogram representing thedistribution of detected photons. Based on the histogram of photondistribution, the response of tissue to light pulses may be determinedin order to study the detected neural activity and/or other attributesof the brain.

A photodetector capable of detecting a single photon (i.e., a singleparticle of optical energy) is an example of a non-invasive detectorthat can be used in an optical measurement system to detect neuralactivity within the brain. An exemplary photodetector is implemented bya semiconductor-based single-photon avalanche diode (SPAD), which iscapable of capturing individual photons with very high time-of-arrivalresolution (a few tens of picoseconds).

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate various embodiments and are a partof the specification. The illustrated embodiments are merely examplesand do not limit the scope of the disclosure. Throughout the drawings,identical or similar reference numbers designate identical or similarelements.

FIG. 1 shows an exemplary optical measurement system.

FIG. 2 illustrates an exemplary detector architecture.

FIG. 3 illustrates an exemplary timing diagram for performing an opticalmeasurement operation using an optical measurement system.

FIG. 4 illustrates a graph of an exemplary temporal point spreadfunction that may be generated by an optical measurement system inresponse to a light pulse.

FIG. 5 shows an exemplary non-invasive wearable brain interface system.

FIG. 6 shows an exemplary optical measurement system.

FIG. 7 shows an illustrative modular assembly.

FIGS. 8A-8B show an exemplary implementation of the modular assembly ofFIG. 7.

FIG. 9 shows an exemplary optical measurement system.

FIG. 10 shows an exemplary circuit of an optical measurement system.

FIG. 11 shows a graph illustrating an exemplary voltage waveform for aphotodetector.

FIG. 12 shows another exemplary circuit of an optical measurementsystem.

FIGS. 13-18 illustrate embodiments of a wearable device that includeselements of the optical detection systems described herein.

FIG. 19 illustrates an exemplary computing device.

FIG. 20 illustrates an exemplary method.

DETAILED DESCRIPTION

In accordance with the systems, circuits, and methods described herein,an optical measurement system may include a control circuit that appliesa bias voltage and a current to photodetectors of the opticalmeasurement system to provide a consistent overvoltage to thephotodetectors. The bias voltage may be applied to a first terminal ofeach photodetector and the current to a second terminal of eachphotodetector. The current may be applied for a predetermined amount oftime to deliver or draw a predetermined amount of charge such that avoltage difference across each photodetector is a consistent voltageamount. The consistent voltage difference may be configured to be set tothe overvoltage, so that the photodetectors may have a consistentsensitivity.

Achieving a consistent sensitivity across photodetectors of the opticalmeasurement system may allow the optical measurement system to generatemore accurate histograms that combine outputs of the photodetectors.These and other advantages and benefits of the present systems,circuits, and methods are described more fully herein.

FIG. 1 shows an exemplary optical measurement system 100 configured toperform an optical measurement operation with respect to a body 102.Optical measurement system 100 may, in some examples, be portable and/orwearable by a user. Optical measurement systems that may be used inconnection with the embodiments described herein are described morefully in U.S. patent application Ser. No. 17/176,315, filed Feb. 16,2021; U.S. patent application Ser. No. 17/176,309, filed Feb. 16, 2021;U.S. patent application Ser. No. 17/176,460, filed Feb. 16, 2021; U.S.patent application Ser. No. 17/176,470, filed Feb. 16, 2021; U.S. patentapplication Ser. No. 17/176,487, filed Feb. 16, 2021; U.S. patentapplication Ser. No. 17/176,539, filed Feb. 16, 2021; U.S. patentapplication Ser. No. 17/176,560, filed Feb. 16, 2021; and U.S. patentapplication Ser. No. 17/176,466, filed Feb. 16, 2021, which applicationsare incorporated herein by reference in their entirety.

In some examples, optical measurement operations performed by opticalmeasurement system 100 are associated with a time domain-based opticalmeasurement technique. Example time domain-based optical measurementtechniques include, but are not limited to, TCSPC, time domain nearinfrared spectroscopy (TD-NIRS), time domain diffusive correlationspectroscopy (TD-DCS), and time domain Digital Optical Tomography(TD-DOT).

As shown, optical measurement system 100 includes a detector 104 thatincludes a plurality of individual photodetectors (e.g., photodetector106), a processor 108 coupled to detector 104, a light source 110, acontroller 112, and optical conduits 114 and 116 (e.g., light pipes).However, one or more of these components may not, in certainembodiments, be considered to be a part of optical measurement system100. For example, in implementations where optical measurement system100 is wearable by a user, processor 108 and/or controller 112 may insome embodiments be separate from optical measurement system 100 and notconfigured to be worn by the user.

Detector 104 may include any number of photodetectors 106 as may serve aparticular implementation, such as 2^(n) photodetectors (e.g., 256, 512,. . . , 16384, etc.), where n is an integer greater than or equal to one(e.g., 4, 5, 8, 10, 11, 14, etc.). Photodetectors 106 may be arranged inany suitable manner.

Photodetectors 106 may each be implemented by any suitable circuitconfigured to detect individual photons of light incident uponphotodetectors 106. For example, each photodetector 106 may beimplemented by a single photon avalanche diode (SPAD) circuit and/orother circuitry as may serve a particular implementation.

Processor 108 may be implemented by one or more physical processing(e.g., computing) devices. In some examples, processor 108 may executeinstructions (e.g., software) configured to perform one or more of theoperations described herein.

Light source 110 may be implemented by any suitable component configuredto generate and emit light. For example, light source 110 may beimplemented by one or more laser diodes, distributed feedback (DFB)lasers, super luminescent diodes (SLDs), light emitting diodes (LEDs),diode-pumped solid-state (DPSS) lasers, super luminescent light emittingdiodes (sLEDs), vertical-cavity surface-emitting lasers (VCSELs),titanium sapphire lasers, micro light emitting diodes (mLEDs), and/orany other suitable laser or light source. In some examples, the lightemitted by light source 110 is high coherence light (e.g., light thathas a coherence length of at least 5 centimeters) at a predeterminedcenter wavelength.

Light source 110 is controlled by controller 112, which may beimplemented by any suitable computing device (e.g., processor 108),integrated circuit, and/or combination of hardware and/or software asmay serve a particular implementation. In some examples, controller 112is configured to control light source 110 by turning light source 110 onand off and/or setting an intensity of light generated by light source110. Controller 112 may be manually operated by a user, or may beprogrammed to control light source 110 automatically.

Light emitted by light source 110 may travel via an optical conduit 114(e.g., a light pipe, a light guide, a waveguide, a single-mode opticalfiber, and/or or a multi-mode optical fiber) to body 102 of a subject.In cases where optical conduit 114 is implemented by a light guide, thelight guide may be spring loaded and/or have a cantilever mechanism toallow for conformably pressing the light guide firmly against body 102.

Body 102 may include any suitable turbid medium. For example, in someimplementations, body 102 is a head or any other body part of a human orother animal. Alternatively, body 102 may be a non-living object. Forillustrative purposes, it will be assumed in the examples providedherein that body 102 is a human head.

As indicated by arrow 120, the light emitted by light source 110 entersbody 102 at a first location 122 on body 102. Accordingly, a distal endof optical conduit 114 may be positioned at (e.g., right above, inphysical contact with, or physically attached to) first location 122(e.g., to a scalp of the subject). In some examples, the light mayemerge from optical conduit 114 and spread out to a certain spot size onbody 102 to fall under a predetermined safety limit. At least a portionof the light indicated by arrow 120 may be scattered within body 102.

As used herein, “distal” means nearer, along the optical path of thelight emitted by light source 110 or the light received by detector 104,to the target (e.g., within body 102) than to light source 110 ordetector 104. Thus, the distal end of optical conduit 114 is nearer tobody 102 than to light source 110, and the distal end of optical conduit116 is nearer to body 102 than to detector 104. Additionally, as usedherein, “proximal” means nearer, along the optical path of the lightemitted by light source 110 or the light received by detector 104, tolight source 110 or detector 104 than to body 102. Thus, the proximalend of optical conduit 114 is nearer to light source 110 than to body102, and the proximal end of optical conduit 116 is nearer to detector104 than to body 102.

As shown, the distal end of optical conduit 116 (e.g., a light pipe, alight guide, a waveguide, a single-mode optical fiber, and/or amulti-mode optical fiber) is positioned at (e.g., right above, inphysical contact with, or physically attached to) output location 126 onbody 102. In this manner, optical conduit 116 may collect at least aportion of the scattered light (indicated as light 124) as it exits body102 at location 126 and carry light 124 to detector 104. Light 124 maypass through one or more lenses and/or other optical elements (notshown) that direct light 124 onto each of the photodetectors 106included in detector 104.

Photodetectors 106 may be connected in parallel in detector 104. Anoutput of each of photodetectors 106 may be accumulated to generate anaccumulated output of detector 104. Processor 108 may receive theaccumulated output and determine, based on the accumulated output, atemporal distribution of photons detected by photodetectors 106.Processor 108 may then generate, based on the temporal distribution, ahistogram representing a light pulse response of a target (e.g., braintissue, blood flow, etc.) in body 102. Example embodiments ofaccumulated outputs are described herein.

FIG. 2 illustrates an exemplary detector architecture 200 that may beused in accordance with the systems and methods described herein. Asshown, architecture 200 includes a SPAD circuit 202 that implementsphotodetector 106, a control circuit 204, a time-to-digital converter(TDC) 206, and a signal processing circuit 208. Architecture 200 mayinclude additional or alternative components as may serve a particularimplementation.

In some examples, SPAD circuit 202 may include a SPAD and a fast gatingcircuit configured to operate together to detect a photon incident uponthe SPAD. As described herein, SPAD circuit 202 may generate an outputwhen SPAD circuit 202 detects a photon.

The fast gating circuit included in SPAD circuit 202 may be implementedin any suitable manner. For example, the fast gating circuit may beimplemented by an active voltage source, a capacitor that is pre-chargedwith a bias voltage before a command is provided to arm the SPAD, and/orin any other suitable manner.

In some alternative configurations, SPAD circuit 202 does not include afast gating circuit. In these configurations, the SPAD included in SPADcircuit 202 may be gated in any suitable manner or be configured tooperate in a free running mode with passive quenching.

Control circuit 204 may be implemented by an application specificintegrated circuit (ASIC) or any other suitable circuit configured tocontrol an operation of various components within SPAD circuit 202. Forexample, control circuit 204 may output control logic that puts the SPADincluded in SPAD circuit 202 in either an armed or a disarmed state.

In some examples, control circuit 204 may control an arming and adisarming of a SPAD included in SPAD circuit 202. Control circuit 204may also control a programmable gate width, which specifies how long theSPAD is kept in an armed state before being disarmed.

Control circuit 204 is further configured to control signal processingcircuit 208. For example, control circuit 204 may provide histogramparameters (e.g., time bins, number of light pulses, type of histogram,etc.) to signal processing circuit 208. Signal processing circuit 208may generate histogram data in accordance with the histogram parameters.In some examples, control circuit 204 is at least partially implementedby controller 112.

TDC 206 is configured to measure a time difference between an occurrenceof an output pulse generated by SPAD circuit 202 and an occurrence of alight pulse. To this end, TDC 206 may also receive the same light pulsetiming information that control circuit 204 receives. TDC 206 may beimplemented by any suitable circuitry as may serve a particularimplementation.

Signal processing circuit 208 is configured to perform one or moresignal processing operations on data output by TDC 206. For example,signal processing circuit 208 may generate histogram data based on thedata output by TDC 206 and in accordance with histogram parametersprovided by control circuit 204. To illustrate, signal processingcircuit 208 may generate, store, transmit, compress, analyze, decode,and/or otherwise process histograms based on the data output by TDC 206.In some examples, signal processing circuit 208 may provide processeddata to control circuit 204, which may use the processed data in anysuitable manner. In some examples, signal processing circuit 208 is atleast partially implemented by processor 108.

In some examples, each photodetector 106 (e.g., SPAD circuit 202) mayhave a dedicated TDC 206 associated therewith. For example, for an arrayof N photodetectors 106, there may be a corresponding array of N TDCs206. Likewise, a single control circuit 204 and a single signalprocessing circuit 208 may be provided for a one or more photodetectors106 and/or TDCs 206.

FIG. 3 illustrates an exemplary timing diagram 300 for performing anoptical measurement operation using optical measurement system 100.Optical measurement system 100 may be configured to perform the opticalmeasurement operation by directing light pulses (e.g., laser pulses)toward a target within a body (e.g., body 102). The light pulses may beshort (e.g., 10-2000 picoseconds (ps)) and repeated at a high frequency(e.g., between 100,000 hertz (Hz) and 100 megahertz (MHz)). The lightpulses may be scattered by the target and then detected by opticalmeasurement system 100. Optical measurement system 100 may measure atime relative to the light pulse for each detected photon. By countingthe number of photons detected at each time relative to each light pulserepeated over a plurality of light pulses, optical measurement system100 may generate a histogram that represents a light pulse response ofthe target (e.g., a temporal point spread function (TPSF)). The termshistogram and TPSF are used interchangeably herein to refer to a lightpulse response of a target.

For example, timing diagram 300 shows a sequence of light pulses 302(e.g., light pulses 302-1 and 302-2) that may be applied to the target(e.g., tissue within a brain of a user, blood flow, a fluorescentmaterial used as a probe in a body of a user, etc.). Timing diagram 300also shows a pulse wave 304 representing predetermined gated timewindows (also referred as gated time periods) during whichphotodetectors 106 are gated ON (i.e., armed) to detect photons.Referring to light pulse 302-1, light pulse 302-1 is applied at a timeto. At a time t₁, a first instance of the predetermined gated timewindow begins. Photodetectors 106 may be armed at time t₁, enablingphotodetectors 106 to detect photons scattered by the target during thepredetermined gated time window. In this example, time t₁ is set to beat a certain time after time to, which may minimize photons detecteddirectly from the laser pulse, before the laser pulse reaches thetarget. However, in some alternative examples, time t₁ is set to beequal to time to.

At a time t₂, the predetermined gated time window ends. In someexamples, photodetectors 106 may be disarmed at time t₂. In otherexamples, photodetectors 106 may be reset (e.g., disarmed and re-armed)at time t₂ or at a time subsequent to time t₂. During the predeterminedgated time window, photodetectors 106 may detect photons scattered bythe target. Photodetectors 106 may be configured to remain armed duringthe predetermined gated time window such that photodetectors 106maintain an output upon detecting a photon during the predeterminedgated time window. For example, a photodetector 106 may detect a photonat a time t₃, which is during the predetermined gated time windowbetween times t₁ and t₂. The photodetector 106 may be configured toprovide an output indicating that the photodetector 106 has detected aphoton. The photodetector 106 may be configured to continue providingthe output until time t₂, when the photodetector may be disarmed and/orreset. Optical measurement system 100 may generate an accumulated outputfrom the plurality of photodetectors. Optical measurement system 100 maysample the accumulated output to determine times at which photons aredetected by photodetectors 106 to generate a TPSF.

As described herein, the systems, circuits, and methods described hereinmay obviate the need for the gated time windows described in connectionwith FIG. 3, thereby obviating the need for fast gating circuitry to beincluded in optical measurement system 100.

As mentioned, in some alternative examples, photodetector 106 may beconfigured to operate in a free-running mode such that photodetector 106is not actively armed and disarmed (e.g., at the end of eachpredetermined gated time window represented by pulse wave 304). Incontrast, while operating in the free-running mode, photodetector 106may be configured to reset within a configurable time period after anoccurrence of a photon detection event (i.e., after photodetector 106detects a photon) and immediately begin detecting new photons. However,only photons detected within a desired time window (e.g., during eachgated time window represented by pulse wave 304) may be included in theTPSF.

FIG. 4 illustrates a graph 400 of an exemplary TPSF 402 that may begenerated by optical measurement system 100 in response to a light pulse404 (which, in practice, represents a plurality of light pulses). Graph400 shows a normalized count of photons on a y-axis and time bins on anx-axis. As shown, TPSF 402 is delayed with respect to a temporaloccurrence of light pulse 404. In some examples, the number of photonsdetected in each time bin subsequent to each occurrence of light pulse404 may be aggregated (e.g., integrated) to generate TPSF 402. TPSF 402may be analyzed and/or processed in any suitable manner to determine orinfer detected neural activity.

Optical measurement system 100 may be implemented by or included in anysuitable device. For example, optical measurement system 100 may beincluded, in whole or in part, in a non-invasive wearable device (e.g.,a headpiece) that a user may wear to perform one or more diagnostic,imaging, analytical, and/or consumer-related operations. Thenon-invasive wearable device may be placed on a user's head or otherpart of the user to detect neural activity. In some examples, suchneural activity may be used to make behavioral and mental stateanalysis, awareness and predictions for the user.

Mental state described herein refers to the measured neural activityrelated to physiological brain states and/or mental brain states, e.g.,joy, excitement, relaxation, surprise, fear, stress, anxiety, sadness,anger, disgust, contempt, contentment, calmness, focus, attention,approval, creativity, positive or negative reflections/attitude onexperiences or the use of objects, etc. Further details on the methodsand systems related to a predicted brain state, behavior, preferences,or attitude of the user, and the creation, training, and use of neuromescan be found in U.S. Provisional Patent Application No. 63/047,991,filed Jul. 3, 2020. Exemplary measurement systems and methods usingbiofeedback for awareness and modulation of mental state are describedin more detail in U.S. patent application Ser. No. 16/364,338, filedMar. 26, 2019, published as US2020/0196932A1. Exemplary measurementsystems and methods used for detecting and modulating the mental stateof a user using entertainment selections, e.g., music, film/video, aredescribed in more detail in U.S. patent application Ser. No. 16/835,972,filed Mar. 31, 2020, published as US2020/0315510A1. Exemplarymeasurement systems and methods used for detecting and modulating themental state of a user using product formulation from, e.g., beverages,food, selective food/drink ingredients, fragrances, and assessment basedon product-elicited brain state measurements are described in moredetail in U.S. patent application Ser. No. 16/853,614, filed Apr. 20,2020, published as US2020/0337624A1. Exemplary measurement systems andmethods used for detecting and modulating the mental state of a userthrough awareness of priming effects are described in more detail inU.S. patent application Ser. No. 16/885,596, filed May 28, 2020,published as US2020/0390358A1. These applications and corresponding U.S.publications are incorporated herein by reference in their entirety.

FIG. 5 shows an exemplary non-invasive wearable brain interface system500 (“brain interface system 500”) that implements optical measurementsystem 100 (shown in FIG. 1). As shown, brain interface system 500includes a head-mountable component 502 configured to be attached to auser's head. Head-mountable component 502 may be implemented by a capshape that is worn on a head of a user. Alternative implementations ofhead-mountable component 502 include helmets, beanies, headbands, otherhat shapes, or other forms conformable to be worn on a user's head, etc.Head-mountable component 502 may be made out of any suitable cloth, softpolymer, plastic, hard shell, and/or any other suitable material as mayserve a particular implementation. Examples of headgears used withwearable brain interface systems are described more fully in U.S. Pat.No. 10,340,408, incorporated herein by reference in its entirety.

Head-mountable component 502 includes a plurality of detectors 504,which may implement or be similar to detector 104, and a plurality oflight sources 506, which may be implemented by or be similar to lightsource 110. It will be recognized that in some alternative embodiments,head-mountable component 502 may include a single detector 504 and/or asingle light source 506.

Brain interface system 500 may be used for controlling an optical pathto the brain and for transforming photodetector measurements into anintensity value that represents an optical property of a target withinthe brain. Brain interface system 500 allows optical detection of deepanatomical locations beyond skin and bone (e.g., skull) by extractingdata from photons originating from light source 506 and emitted to atarget location within the user's brain, in contrast to conventionalimaging systems and methods (e.g., optical coherence tomography (OCT)),which only image superficial tissue structures or through opticallytransparent structures.

Brain interface system 500 may further include a processor 508configured to communicate with (e.g., control and/or receive signalsfrom) detectors 504 and light sources 506 by way of a communication link510. Communication link 510 may include any suitable wired and/orwireless communication link. Processor 508 may include any suitablehousing and may be located on the user's scalp, neck, shoulders, chest,or arm, as may be desirable. In some variations, processor 508 may beintegrated in the same assembly housing as detectors 504 and lightsources 506.

As shown, brain interface system 500 may optionally include a remoteprocessor 512 in communication with processor 508. For example, remoteprocessor 512 may store measured data from detectors 504 and/orprocessor 508 from previous detection sessions and/or from multiplebrain interface systems (not shown). Power for detectors 504, lightsources 506, and/or processor 508 may be provided via a wearable battery(not shown). In some examples, processor 508 and the battery may beenclosed in a single housing, and wires carrying power signals fromprocessor 508 and the battery may extend to detectors 504 and lightsources 506. Alternatively, power may be provided wirelessly (e.g., byinduction).

In some alternative embodiments, head mountable component 502 does notinclude individual light sources. Instead, a light source configured togenerate the light that is detected by detector 504 may be includedelsewhere in brain interface system 500. For example, a light source maybe included in processor 508 and coupled to head mountable component 502through optical connections.

Optical measurement system 100 may alternatively be included in anon-wearable device (e.g., a medical device and/or consumer device thatis placed near the head or other body part of a user to perform one ormore diagnostic, imaging, and/or consumer-related operations). Opticalmeasurement system 100 may alternatively be included in a sub-assemblyenclosure of a wearable invasive device (e.g., an implantable medicaldevice for brain recording and imaging).

FIG. 6 shows an exemplary optical measurement system 600 in accordancewith the principles described herein. Optical measurement system 600 maybe an implementation of optical measurement system 100 and, as shown,includes a wearable assembly 602, which includes N light sources 604(e.g., light sources 604-1 through 604-N) and M detectors 606 (e.g.,detectors 606-1 through 606-M). Optical measurement system 600 mayinclude any of the other components of optical measurement system 100 asmay serve a particular implementation. N and M may each be any suitablevalue (i.e., there may be any number of light sources 604 and detectors606 included in optical measurement system 600 as may serve a particularimplementation).

Light sources 604 are each configured to emit light (e.g., a sequence oflight pulses) and may be implemented by any of the light sourcesdescribed herein. Detectors 606 may each be configured to detect arrivaltimes for photons of the light emitted by one or more light sources 604after the light is scattered by the target. For example, a detector 606may include a photodetector configured to generate a photodetectoroutput pulse in response to detecting a photon of the light and a TDCconfigured to record a timestamp symbol in response to an occurrence ofthe photodetector output pulse, the timestamp symbol representative ofan arrival time for the photon (i.e., when the photon is detected by thephotodetector).

Wearable assembly 602 may be implemented by any of the wearable devices,modular assemblies, and/or wearable units described herein. For example,wearable assembly 602 may be implemented by a wearable device (e.g.,headgear) configured to be worn on a user's head. Wearable assembly 602may additionally or alternatively be configured to be worn on any otherpart of a user's body.

Optical measurement system 600 may be modular in that one or morecomponents of optical measurement system 600 may be removed, changedout, or otherwise modified as may serve a particular implementation. Assuch, optical measurement system 600 may be configured to conform tothree-dimensional surface geometries, such as a user's head. Exemplarymodular multimodal measurement systems are described in more detail inU.S. Provisional patent application Ser. No. 17/176,460, filed Feb. 16,2021, U.S. Provisional patent application Ser. No. 17/176,470, filedFeb. 16, 2021, U.S. Provisional patent application Ser. No. 17/176,487,filed Feb. 16, 2021, U.S. Provisional Patent Application No. 63/038,481,filed Feb. 16, 2021, and U.S. Provisional patent application Ser. No.17/176,560, filed Feb. 16, 2021, which applications are incorporatedherein by reference in their respective entireties.

FIG. 7 shows an illustrative modular assembly 700 that may implementoptical measurement system 600. Modular assembly 700 is illustrative ofthe many different implementations of optical measurement system 600that may be realized in accordance with the principles described herein.

As shown, modular assembly 700 includes a plurality of modules 702(e.g., modules 702-1 through 702-3). While three modules 702 are shownto be included in modular assembly 700, in alternative configurations,any number of modules 702 (e.g., a single module up to sixteen or moremodules) may be included in modular assembly 700.

Each module 702 includes a light source (e.g., light source 704-1 ofmodule 702-1 and light source 704-2 of module 702-2) and a plurality ofdetectors (e.g., detectors 706-1 through 706-6 of module 702-1). In theparticular implementation shown in FIG. 7, each module 702 includes asingle light source and six detectors. Each light source is labeled “S”and each detector is labeled “D”.

Each light source depicted in FIG. 7 may be implemented by one or morelight sources similar to light source 110 and may be configured to emitlight directed at a target (e.g., the brain).

Each light source depicted in FIG. 7 may be located at a center regionof a surface of the light source's corresponding module. For example,light source 704-1 is located at a center region of a surface 708 ofmodule 702-1. In alternative implementations, a light source of a modulemay be located away from a center region of the module.

Each detector depicted in FIG. 7 may implement or be similar to detector104 and may include a plurality of photodetectors (e.g., SPADs) as wellas other circuitry (e.g., TDCs), and may be configured to detect arrivaltimes for photons of the light emitted by one or more light sourcesafter the light is scattered by the target.

The detectors of a module may be distributed around the light source ofthe module. For example, detectors 706 of module 702-1 are distributedaround light source 704-1 on surface 708 of module 702-1. In thisconfiguration, detectors 706 may be configured to detect photon arrivaltimes for photons included in light pulses emitted by light source704-1. In some examples, one or more detectors 706 may be close enoughto other light sources to detect photon arrival times for photonsincluded in light pulses emitted by the other light sources. Forexample, because detector 706-3 is adjacent to module 702-2, detector706-3 may be configured to detect photon arrival times for photonsincluded in light pulses emitted by light source 704-2 (in addition todetecting photon arrival times for photons included in light pulsesemitted by light source 704-1).

In some examples, the detectors of a module may all be equidistant fromthe light source of the same module. In other words, the spacing betweena light source (i.e., a distal end portion of a light source opticalconduit) and the detectors (i.e., distal end portions of opticalconduits for each detector) are maintained at the same fixed distance oneach module to ensure homogeneous coverage over specific areas and tofacilitate processing of the detected signals. The fixed spacing alsoprovides consistent spatial (lateral and depth) resolution across thetarget area of interest, e.g., brain tissue. Moreover, maintaining aknown distance between the light source, e.g., light emitter, and thedetector allows subsequent processing of the detected signals to inferspatial (e.g., depth localization, inverse modeling) information aboutthe detected signals. Detectors of a module may be alternativelydisposed on the module as may serve a particular implementation.

In FIG. 7, modules 702 are shown to be adjacent to and touching oneanother. Modules 702 may alternatively be spaced apart from one another.For example, FIGS. 8A-8B show an exemplary implementation of modularassembly 700 in which modules 702 are configured to be inserted intoindividual slots 802 (e.g., slots 802-1 through 802-3, also referred toas cutouts) of a wearable assembly 804. In particular, FIG. 8A shows theindividual slots 802 of the wearable assembly 804 before modules 702have been inserted into respective slots 802, and FIG. 8B shows wearableassembly 804 with individual modules 702 inserted into respectiveindividual slots 802.

Wearable assembly 804 may implement wearable assembly 602 and may beconfigured as headgear and/or any other type of device configured to beworn by a user.

As shown in FIG. 8A, each slot 802 is surrounded by a wall (e.g., wall806) such that when modules 702 are inserted into their respectiveindividual slots 802, the walls physically separate modules 702 one fromanother. In alternative embodiments, a module (e.g., module 702-1) maybe in at least partial physical contact with a neighboring module (e.g.,module 702-2).

Each of the modules described herein may be inserted into appropriatelyshaped slots or cutouts of a wearable assembly, as described inconnection with FIGS. 8A-8B. However, for ease of explanation, suchwearable assemblies are not shown in the figures.

As shown in FIGS. 7 and 8B, modules 702 may have a hexagonal shape.Modules 702 may alternatively have any other suitable geometry (e.g., inthe shape of a pentagon, octagon, square, rectangular, circular,triangular, free-form, etc.).

FIG. 9 shows an exemplary optical measurement system 900, which may bean implementation or a portion of optical measurement system 100.Optical measurement system 900 includes a control circuit 902 (e.g., animplementation or portion of control circuit 204) coupled to an array ofphotodetectors 904 (e.g., photodetectors 904-1 through 904-N), which maybe implementations of photodetector 106, SPAD circuit 202, etc. Opticalmeasurement system 900 further includes a light source 906 (e.g., animplementation of light source 110). Light source 906 may be configuredto emit light directed toward a target 908 (e.g., a body of a user), asshown by an arrow 910. Photodetector 904 may be configured to detect oneor more photons of the light after the light is scattered by target 908.

Control circuit 902 may be configured to output, to photodetector 904, abias voltage 912 to arm photodetector 904 to detect photons. Biasvoltage 912 may be configured to have a voltage level that is apredetermined voltage level higher than a breakdown voltage ofphotodetector 904. The difference between bias voltage 912 and thebreakdown voltage may define an overvoltage for photodetector 904. Theovervoltage may affect the sensitivity of photodetector 904, as settingbias voltage 912 to be higher than the breakdown voltage by a particularovervoltage allows for an electric field that is primed for an avalancheto occur in response to detecting a single photon. However, due toprocess variations, the breakdown voltage of photodetectors 904 mayvary. As a result, while bias voltage 912 may be set to a voltage levelthat is configured to be a particular overvoltage higher than thebreakdown voltage, an actual overvoltage may vary among photodetectors904. The varying overvoltage may result in varying sensitivity (e.g.,photon detection probability) among photodetectors 904. Such varyingsensitivity may affect histograms generated based on combining outputsfrom photodetectors 904.

Control circuit 902 may compensate for such variations amongphotodetectors 904 by providing a current 914 along with bias voltage912 to photodetectors 904. Control circuit 902 may be configured toprovide bias voltage 912 to a first terminal of each photodetector 904and current 914 to a second terminal of each photodetector 904. Forexample, control circuit 902 may provide bias voltage 912 to a cathodeof each photodetector 904 and current 914 to an anode of eachphotodetector 904. Current 914 may be configured to discharge the secondterminal (e.g., the anode) of each photodetector 904 a fixed amount sothat a voltage across each photodetector 904 may be a consistentpredetermined overvoltage level for photodetectors 904.

Additionally or alternatively, control circuit 902 may provide biasvoltage 912 to the anode of each photodetector 904 and current 914 tothe cathode of each photodetector 904. In this configuration, current914 may be configured to charge the second terminal (e.g., the cathode)of photodetector 904 a fixed amount so that the voltage acrossphotodetector 904 may be the consistent predetermined overvoltage levelfor photodetectors 904.

In either configuration, the consistent overvoltage for photodetectors904 may allow optical measurement system 900 to maintain a consistentsensitivity across photodetectors 904, which may allow for accuratehistograms generated based on outputs of photodetectors 904 and/orprovide other advantages and benefits described herein.

FIG. 10 shows an exemplary circuit 1000, which may be a portion ofoptical measurement system 900, such as an implementation or portion ofcontrol circuit 902 coupled to photodetector 904. Circuit 1000 includesa first terminal (e.g., a cathode 1002) of photodetector 904 receivingbias voltage 912. Circuit 1000 further includes a second terminal (e.g.,an anode 1004) of photodetector 904 configured to receive current 914via a current source 1006. Current source 1006 may be implemented in anysuitable manner, including any suitable component and/or circuit thatprovides a current.

Circuit 1000 further includes a first transistor 1008, which may beconfigured to act as a switch to selectively couple current source 1006to anode 1004 of photodetector 904 via a second transistor 1010 and athird transistor 1012. Second transistor 1010 and third transistor 1012may together be configured to act as a current mirror so that anode 1004may be discharged toward ground when transistor 1008 is on and currentsource 1006 is coupled to anode 1004.

Current source 1006 may be configured to deliver charge for apredetermined period of time so that a voltage level of anode 1004 isdrawn toward ground a predetermined voltage amount by way of the currentmirror. The predetermined period of time may be set so that thepredetermined voltage amount that the voltage level of anode 1004 dropsequals the overvoltage for photodetector 904. In this manner, aconsistent overvoltage may be maintained for photodetector 904.

FIG. 11 shows a graph 1100 illustrating an exemplary waveform 1102representing a voltage level over time for anode 1004. For a firstportion 1104 of waveform 1102, the voltage level for anode 1004 is drawnall the way down to ground (e.g., 0 volts). Drawing anode 1004 down toground may allow for an initial reset phase of optical measurementsystem 900.

At time to, photodetector 904 may detect a photon. As a result,photodetector 904 may avalanche and charge anode 1004 to raise thevoltage level of anode 1004 to equal a voltage level of a bias voltage(e.g., bias voltage 912) applied to photodetector 904 minus a breakdownvoltage of photodetector 904. A voltage level 1106 may represent thisvoltage level (bias voltage 912 minus the breakdown voltage ofphotodetector 904). At time t₁, a current (e.g., current 914) is appliedto anode 1004 for a predetermined amount of time, shown as a duration1108 between time t₁ and time t₂. For instance, transistor 1008 may beturned on for duration 1108 so that current source 1006 may be coupledto a current mirror (e.g., transistors 1010 and 1012) coupled to anode1004. The current may be applied to anode 1004 for duration 1108 via thecurrent mirror, so that anode 1004 is discharged to draw the voltagelevel of anode 1004 down a predetermined voltage amount. A voltage level1110 may represent the voltage level of anode 1004 after current 914 isapplied for duration 1108. A difference between voltage level 1106 andvoltage level 1110 is shown by a voltage difference 1112, which mayrepresent the predetermined voltage amount, which may be an overvoltagefor photodetector 904.

As described, the breakdown voltage of photodetectors 904 may vary fromphotodetector to photodetector. Consequently, voltage level 1106 mayvary among photodetectors 904, as voltage level 1106 is defined by biasvoltage 912 (which may be uniform or substantially uniform acrossphotodetectors 904) minus the breakdown voltage (which may vary).However, as current 914 may be a uniform or substantially uniformcurrent applied for a uniform predetermined amount of time, anode 1004may be discharged a uniform amount, resulting in a consistentovervoltage across photodetectors 904.

FIG. 12 shows another exemplary circuit 1200, which may be a portion ofoptical measurement system 900, such as an implementation or portion ofcontrol circuit 902 coupled to photodetector 904. Circuit 1200 may besimilar to circuit 1000 with a reversed polarity of photodetector 904receiving a bias voltage and a current. For example, circuit 1200includes a first terminal (e.g., anode 1004) of photodetector 904receiving bias voltage 912. Circuit 1200 further includes a secondterminal (e.g., cathode 1002) of photodetector 904 configured to receivecurrent 914 via a current source 1006.

Circuit 1200 further includes first transistor 1206, configured to actas a switch to selectively couple current source 1006 to cathode 1002 ofphotodetector 904 via second transistor 1010 and third transistor 1012.Second transistor 1010 and third transistor 1012 may together beconfigured to act as a current mirror so that cathode may be charged apredetermined amount when transistor 1008 is on and current source 1006is coupled to cathode 1002.

Current source 1006 may be configured to deliver charge for apredetermined period of time so that a voltage level of cathode israised a predetermined voltage amount by way of the current mirror. Thepredetermined period of time may be set so that the predeterminedvoltage amount that the voltage level of cathode is raised equals theovervoltage for photodetector 904. In this manner, a consistentovervoltage may be maintained for photodetector 904.

In some examples, the current applied by current source 1006 may beconfigured to be substantially uniform across photodetectors 904 byusing a current source that is shared by the array of photodetectors 904(or a subset of the array of photodetectors 904). Further, transistor1008 and/or transistor 1010 may also be implemented using sharedtransistors for the array of photodetectors 904. By using a sharedcurrent source 1006 and a shared transistor 1008, the current may beapplied to the second terminals of the array of photodetectors 904 at asame current level for a same period of time to each of photodetectors904 in the array. Transistor 1012, on the other hand, may be implementedas a separate transistor for each of photodetectors 904. Alternatively,transistor 1012 may also be implemented as a shared transistor. In otherexamples, current source 1006, transistor 1008, and/or transistor 1010may be implemented as separate components for each of photodetectors904.

FIGS. 13-18 illustrate embodiments of a wearable device 1300 thatincludes elements of the optical detection systems described herein. Inparticular, the wearable devices 1300 shown in FIGS. 13-18 include aplurality of modules 1302, similar to the modules described herein. Forexample, each module 1302 may include a light source (e.g., light source704-1) and a plurality of detectors (e.g., detectors 706-1 through706-6). The wearable devices 1300 may each also include a controller(e.g., controller 112) and a processor (e.g., processor 108) and/or becommunicatively connected to a controller and processor. In general,wearable device 1300 may be implemented by any suitable headgear and/orclothing article configured to be worn by a user. The headgear and/orclothing article may include batteries, cables, and/or other peripheralsfor the components of the optical measurement systems described herein.

FIG. 13 illustrates an embodiment of a wearable device 1300 in the formof a helmet with a handle 1304. A cable 1306 extends from the wearabledevice 1300 for attachment to a battery or hub (with components such asa processor or the like). FIG. 14 illustrates another embodiment of awearable device 1300 in the form of a helmet showing a back view. FIG.15 illustrates a third embodiment of a wearable device 1300 in the formof a helmet with the cable 1306 leading to a wearable garment 1308 (suchas a vest or partial vest) that can include a battery or a hub.Alternatively or additionally, the wearable device 1300 can include acrest 1310 or other protrusion for placement of the hub or battery.

FIG. 16 illustrates another embodiment of a wearable device 1300 in theform of a cap with a wearable garment 1308 in the form of a scarf thatmay contain or conceal a cable, battery, and/or hub. FIG. 17 illustratesadditional embodiments of a wearable device 1300 in the form of a helmetwith a one-piece scarf 1308 or two-piece scarf 1308-1. FIG. 18illustrates an embodiment of a wearable device 1300 that includes a hood1310 and a beanie 1312 which contains the modules 1302, as well as awearable garment 1308 that may contain a battery or hub.

In some examples, a non-transitory computer-readable medium storingcomputer-readable instructions may be provided in accordance with theprinciples described herein. The instructions, when executed by aprocessor of a computing device, may direct the processor and/orcomputing device to perform one or more operations, including one ormore of the operations described herein. Such instructions may be storedand/or transmitted using any of a variety of known computer-readablemedia.

A non-transitory computer-readable medium as referred to herein mayinclude any non-transitory storage medium that participates in providingdata (e.g., instructions) that may be read and/or executed by acomputing device (e.g., by a processor of a computing device). Forexample, a non-transitory computer-readable medium may include, but isnot limited to, any combination of non-volatile storage media and/orvolatile storage media. Exemplary non-volatile storage media include,but are not limited to, read-only memory, flash memory, a solid-statedrive, a magnetic storage device (e.g. a hard disk, a floppy disk,magnetic tape, etc.), ferroelectric random-access memory (“RAM”), and anoptical disc (e.g., a compact disc, a digital video disc, a Blu-raydisc, etc.). Exemplary volatile storage media include, but are notlimited to, RAM (e.g., dynamic RAM).

FIG. 19 illustrates an exemplary computing device 1900 that may bespecifically configured to perform one or more of the processesdescribed herein. Any of the systems, units, computing devices, and/orother components described herein may be implemented by computing device1900.

As shown in FIG. 19, computing device 1900 may include a communicationinterface 1902, a processor 1904, a storage device 1906, and aninput/output (“I/O”) module 1908 communicatively connected one toanother via a communication infrastructure 1910. While an exemplarycomputing device 1900 is shown in FIG. 19, the components illustrated inFIG. 19 are not intended to be limiting. Additional or alternativecomponents may be used in other embodiments. Components of computingdevice 1900 shown in FIG. 19 will now be described in additional detail.

Communication interface 1902 may be configured to communicate with oneor more computing devices. Examples of communication interface 1902include, without limitation, a wired network interface (such as anetwork interface card), a wireless network interface (such as awireless network interface card), a modem, an audio/video connection,and any other suitable interface.

Processor 1904 generally represents any type or form of processing unitcapable of processing data and/or interpreting, executing, and/ordirecting execution of one or more of the instructions, processes,and/or operations described herein. Processor 1904 may performoperations by executing computer-executable instructions 1912 (e.g., anapplication, software, code, and/or other executable data instance)stored in storage device 1906.

Storage device 1906 may include one or more data storage media, devices,or configurations and may employ any type, form, and combination of datastorage media and/or device. For example, storage device 1906 mayinclude, but is not limited to, any combination of the non-volatilemedia and/or volatile media described herein. Electronic data, includingdata described herein, may be temporarily and/or permanently stored instorage device 1906. For example, data representative ofcomputer-executable instructions 1912 configured to direct processor1904 to perform any of the operations described herein may be storedwithin storage device 1906. In some examples, data may be arranged inone or more databases residing within storage device 1906.

I/O module 1908 may include one or more I/O modules configured toreceive user input and provide user output. I/O module 1908 may includeany hardware, firmware, software, or combination thereof supportive ofinput and output capabilities. For example, I/O module 1908 may includehardware and/or software for capturing user input, including, but notlimited to, a keyboard or keypad, a touchscreen component (e.g.,touchscreen display), a receiver (e.g., an RF or infrared receiver),motion sensors, and/or one or more input buttons.

I/O module 1908 may include one or more devices for presenting output toa user, including, but not limited to, a graphics engine, a display(e.g., a display screen), one or more output drivers (e.g., displaydrivers), one or more audio speakers, and one or more audio drivers. Incertain embodiments, I/O module 1908 is configured to provide graphicaldata to a display for presentation to a user. The graphical data may berepresentative of one or more graphical user interfaces and/or any othergraphical content as may serve a particular implementation.

FIG. 20 illustrates an exemplary method 2000 that may be performed bycontrol circuit 902 and/or any implementation thereof. While FIG. 20illustrates exemplary operations according to one embodiment, otherembodiments may omit, add to, reorder, and/or modify any of theoperations shown in FIG. 20. Each of the operations shown in FIG. 20 maybe performed in any of the ways described herein.

In operation 2002, a control circuit of an optical measurement systemapplies a bias voltage to a first terminal of a photodetector configuredto detect photons from a light pulse after the light pulse reflects offa target, e.g., body of a user.

In operation 2004, the control circuit applies, for a predeterminedamount of time using a current source, a current to a second terminal ofthe photodetector to produce a predetermined voltage difference acrossthe photodetector.

An illustrative optical measurement system includes a light sourceconfigured to emit light directed at a target. The optical measurementsystem further includes a photodetector configured to detect a photon ofthe light after the light is scattered by the target. The opticalmeasurement system further includes a control circuit configured to armthe photodetector by applying a bias voltage to a first terminal of thephotodetector and applying, for a predetermined amount of time using acurrent source, a current to a second terminal of the photodetector toproduce a predetermined voltage difference across the photodetector.

Another illustrative optical measurement system includes a wearablesystem for use by a user. The wearable system includes a head-mountablecomponent configured to be attached to a head of the user, thehead-mountable component including a photodetector configured to detectphotons from a light pulse after the light pulse reflects off a targetwithin the head. The wearable system further includes a control circuitconfigured to arm the photodetector by applying a bias voltage to afirst terminal of the photodetector and applying, for a predeterminedamount of time using a current source, a current to a second terminal ofthe photodetector to produce a predetermined voltage difference acrossthe photodetector.

An exemplary method includes applying, by a control circuit, a biasvoltage to a first terminal of a photodetector configured to detectphotons from a light pulse after the light pulse reflects off a target.The method further includes applying, by the control circuit for apredetermined amount of time using a current source, a current to asecond terminal of the photodetector to produce a predetermined voltagedifference across the photodetector.

In the preceding description, various exemplary embodiments have beendescribed with reference to the accompanying drawings. It will, however,be evident that various modifications and changes may be made thereto,and additional embodiments may be implemented, without departing fromthe scope of the invention as set forth in the claims that follow. Forexample, certain features of one embodiment described herein may becombined with or substituted for features of another embodimentdescribed herein. The description and drawings are accordingly to beregarded in an illustrative rather than a restrictive sense.

1. An optical measurement system comprising: a light source configuredto emit light directed at a target; a photodetector configured to detecta photon of the light after the light is scattered by the target; and acontrol circuit configured to arm the photodetector by: applying a biasvoltage to a first terminal of the photodetector; and applying, for apredetermined amount of time using a current source, a current to asecond terminal of the photodetector to produce a predetermined voltagedifference across the photodetector.
 2. The optical measurement systemof claim 1, wherein the control circuit comprises a current mirrorcoupling the current source to the second terminal of the photodetector.3. The optical measurement system of claim 2, wherein: the firstterminal comprises a cathode of the photodetector; the second terminalcomprises an anode of the photodetector; and the applying the currentcomprises discharging the anode of the photodetector to produce thepredetermined voltage difference across the photodetector.
 4. Theoptical measurement system of claim 2, wherein: the first terminalcomprises an anode of the photodetector; the second terminal comprises acathode of the photodetector; and the applying the current comprisesdelivering charge to the cathode of the photodetector to produce thepredetermined voltage difference across the photodetector.
 5. Theoptical measurement system of claim 1, further comprising an array ofphotodetectors including the photodetector; wherein the control circuitis configured to arm the array of photodetectors by: applying the biasvoltage to first terminals of the array of photodetectors; and applyingthe current using the current source to second terminals of the array ofphotodetectors.
 6. The optical measurement system of claim 5, whereinthe current source is a shared current source configured to apply thecurrent to the second terminals.
 7. The optical measurement system ofclaim 6, further comprising a plurality of current mirrors coupling theshared current source to the second terminals of the array ofphotodetectors.
 8. The optical measurement system of claim 7, whereinthe plurality of current mirrors comprise a shared transistor and aplurality of local transistors each corresponding to a differentphotodetector of the array of photodetectors.
 9. The optical measurementsystem of claim 1, wherein the photodetector comprises: a single photonavalanche diode (SPAD); and a fast gating circuit configured to arm anddisarm the SPAD.
 10. The optical measurement system of claim 1, whereinthe photodetector is included in a wearable device configured to be wornby a user.
 11. The optical measurement system of claim 10, wherein thewearable device includes a head-mountable component configured to beworn on a head of a user.
 12. A wearable system for use by a usercomprising: a head-mountable component configured to be attached to ahead of the user, the head-mountable component comprising aphotodetector configured to detect photons from a light pulse after thelight pulse reflects off a target within the head; and a control circuitconfigured to arm the photodetector by: applying a bias voltage to afirst terminal of the photodetector; and applying, for a predeterminedamount of time using a current source, a current to a second terminal ofthe photodetector to produce a predetermined voltage difference acrossthe photodetector.
 13. The wearable system of claim 12, wherein thecontrol circuit comprises a current mirror coupling the current sourceto the second terminal of the photodetector.
 14. The wearable system ofclaim 13, wherein: the first terminal comprises a cathode of thephotodetector; the second terminal comprises an anode of thephotodetector; and the applying the current comprises discharging theanode of the photodetector to produce the predetermined voltagedifference across the photodetector.
 15. The wearable system of claim13, wherein: the first terminal comprises an anode of the photodetector;the second terminal comprises a cathode of the photodetector; and theapplying the current comprises delivering charge to the cathode of thephotodetector to produce the predetermined voltage difference across thephotodetector.
 16. The wearable system of claim 12, further comprisingan array of photodetectors including the photodetector; wherein thecontrol circuit is configured to arm the array of photodetectors by:applying the bias voltage to first terminals of the array ofphotodetectors; and applying the current using the current source tosecond terminals of the array of photodetectors.
 17. The wearable systemof claim 16, wherein the current source is a shared current sourceconfigured to apply the current to the second terminals.
 18. Thewearable system of claim 17, further comprising a plurality of currentmirrors coupling the shared current source to the second terminals ofthe array of photodetectors.
 19. The wearable system of claim 18,wherein the plurality of current mirrors comprise a shared transistorand a plurality of local transistors each corresponding to a differentphotodetector of the array of photodetectors.
 20. The wearable system ofclaim 12, wherein the photodetector comprises: a single photon avalanchediode (SPAD); and a fast gating circuit configured to arm and disarm theSPAD. 21-28. (canceled)